Well if you do a search on this you will find 1001 posts on this and everyone has their own opinion. (mine of course is right jk. Anyway pine is a bad choice for a lot of reasons that others will most likely elaborate on. Aspen is a little expensive in my opinion but a lot of people use it and love it. Cypress mulch is good as well if you are going for aesthetics, it holds moisture well too. I personally use towels which are white and roughly the size of my tub but I do this due to the fact that we have grown to over 40 balls now and they are awesome to simply pull out throw in the washer with some bleach and reuse. I keep a stack of them handy so there is always plenty. If you wanted to try this route you could probably find several for a dollar at a dollar store. Even at a buck or two each they are pretty cheap considering they aren't disposable. That's what we like. Hope this helps. Have fun and good luck!